STUDIES IN MAMMALIAN EMBRYOLOGY. 517 



mesoblast connected with and originating from the proto- 

 chordal plate^ and that from the annular zone of hypoblast. 



Thus the region bounded by the dotted line in stages 45 

 and 42 (figs. 62 — 64, and 79 — 81) contains mesoblast that has 

 arisen from the three distinct sources above indicated. His- 

 tologically the mesoblast offers no salient points by which, 

 from this stage onward, this different mode of origin could 

 rigorously be detected. Still it deserves special attention, 

 that in the oblique sections through the stage 42 (figs. 79 — 

 81) the difference between protochordal-plate-mesoblast, pro- 

 tochordal-wedge-mesoblast, and gastrula-ridge-mesoblast could 

 yet be distinguished with some precision. Fig. 87 is the 

 most striking example of this, and at the same time teaches 

 us (as do also the figs. 84, 89, and 90) that the protochordal- 

 wedge-mesoblast gives ample evidence of being from the 

 beginning a double plate of cells. 



The circular patch of hypoblast enclosed by the annular 

 ring was yet more distinct in the embryos 73 than it is in the 

 embryos 45. As far as it underlies the gastrula ridge and proto- 

 chordal wedge it undergoes a retrogressive metamorphosis, 

 and can no longer be distinguished as a separate layer. 



It will be of better aid towards the comprehension of the 

 description above given to compare figs. 66 to 78, which repre- 

 sent sections through stage 45, with those of PL XXXIX, 

 than to enter into any further detailed statements. The pre- 

 parations have been very carefully reproduced in the figures 

 that were first sketched with the camera, and for criticism of 

 the views here advanced the original preparations are at the 

 disposal of such investigators as should wish to convince 

 themselves by personal inquiry. 



The passage from stage 45 to stage 42 is a more gradual 

 one than was that from 73 to 45. The mesoblast covers a 

 wider area. Piotochordal plate and wedge now definitely con- 

 stitute an axial strip of tissue, from which the notochord will 

 take its origin when it has become isolated out of the enteric 

 hypoblast, with which it is as yet continuous. The mesoblast, 

 which was originally in direct lateral continuity with these 



